Within the 3rd generation partnership programme (3GPP) the work of standardizing the first release of the 3G long term evolution (LTE) system has been intense during the recent year.
One important focus area in LTE standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is that the new system shall be self optimizing and self configuring in as many aspects as possible. Two related such aspects are automatic assignment of physical cell identities (PCIDs) and automatic construction of neighbour cell relation (NCR) lists.
A PCID is a short identity that is locally unique in the vicinity of the cell. In LTE there are 504 unique such identities defined in the standard and consequently the PCIDs need to be reused in large networks. The base stations (denoted eNBs in 3GPP) transmit a reference signal corresponding to the PCID for the mobile stations (denoted UEs in 3GPP) to measure on. The UEs in LTE uses the reference signals to measure e.g. the reference symbol received power (RSRP) and these measurements are used when performing initial cell selection as well as handovers.
When a connected UE detects a strong reference signal this may trigger the UE to report the measurement, along with the corresponding PCID, to the serving eNB. In case the eNB already has a neighbour relation to a cell with the reported PCID the eNB will associate the measurement to the corresponding cell. The eNB might as a result order the UE to perform e.g. a handover to the reported handover candidate target cell.
In case the serving eNB does not recognize the PCID it may order the UE to read and report back the global cell identity (GID) as well as the network identity (i.e. the public land mobile network identity; PLMNID) of the unknown cell. This information is in LTE transmitted on the physical broadcast channel (PBCH). Once the eNB has obtained knowledge of the globally unique identifier, i.e. the combination of GID and PLMNID of the newly detected neighbouring cell it may contact a central server, e.g. a domain name server (DNS) or similar, and obtain the remaining connectivity information it needs to setup a neighbour cell relation with the target cell.
In case the PCID are assigned without manual planning or without an advanced automatic algorithm there is a risk of PCID conflicts. A PCID conflict may be detected if a cell has two neighbours in the NCR list that:                use the same PCID on the same carrier frequency; and,        that have the same PLMNID but different GID.        
Thus, two cells with conflicting identities need help from a third common neighbouring cell to detect the conflict. As a result, one or both cells must change its old colliding PCID to a non-colliding PCID.
Most of the PCID collisions manifest themselves as ambiguities in NCR lists. However, in case of a small isolated micro cell inside a larger macro cell there may not be any third cell that has both the micro cell and the macro cells as neighbours.
Another case where it is not possible to rely on a third neighbouring cell to detect a PCID conflict is, if there are two isolated cells, e.g. on an island with only two cells.
Yet another case is when there is a “highway deployment” where several cells are aligned along a road such that each cell only has two neighbours.
To be able to detect PCID conflict also in difficult situations as in the examples provided above, the concept of PCID transmission gaps has been introduced. The idea with the PCID transmission gap is to stop transmitting the reference signals corresponding to the PCID at predefined (e.g. pseudo-random) times and to order the connected UEs to try to detect during the gap if any other cell is transmitting reference symbols corresponding to the same PCID. Note, that during normal operation there is in general no need to issue a reference symbol transmission gap. The PCID is a static property of a cell that is initialized when the cell first goes into operation and then that value is typically never changed during the lifetime of the cell. Reference symbol transmission gaps are therefore most useful in newly installed cells that need to verify that the initial PCID is locally non-colliding. Reference symbol transmission gaps could however also be used infrequently (e.g. once a day) to verify that the used PCID is still locally non-colliding. Especially in small micro cells with few neighbouring cells this might be a good safety measure.
Note however, that it is not sufficient to issue reference symbol transmission gaps when there are no, or very few, UEs in the cell, e.g. night time, since this solution must rely on the UEs to measure and detect potential PCID conflicts. Hence the PCID collision detection algorithm using reference symbol transmission gap should preferably be executed during operation hours with normal traffic, e.g. day time.
As mentioned above, transmission gaps of the reference symbols may be used to detect physical cell identity collisions. This is needed e.g. in order for an isolated small micro cell to detect that the PCID is colliding with that of a larger macro cell.
However, if this transmission gap is done by simply avoiding transmission of reference symbols in certain slots or sub-frames then there are some negative consequences:                First of all, no data may be transmitted from a cell that issues a reference symbol transmission gap. Also, since the reference symbols are used for calculating the channel estimates needed for coherent demodulation, it is not possible to transmit any control information during the gap, such as uplink grants to the UEs. This may lead to the unwanted side effect that the uplink resource will become unused at a future uplink sub-frame, simply because no UEs are granted to transmit on any uplink resources for that corresponding uplink sub-frame.        UEs in neighbouring cells performing RSRP measurements on a cell that issues a reference symbol transmission gap will experience reduced measurement accuracy. Since they are not aware of when the non-serving cell will stop transmission of the reference symbols they will observe a sudden drop in unfiltered RSRP measurements. Typically the RSRP measurements are filtered (e.g. a recursive low-pass filtering with forgetting factor α) and a sudden noise transient caused by a reference symbol transmission gap may affect the filtered RSRP measurement for a considerable time. This may have a negative affect on the handover performance.        In case RSRP measurements are used also for e.g. inter-cell interference coordination (ICIC) purposes then the potential gains of such algorithms (and other RRM related algorithms relying on accurate RSRP measurements) may be reduced.        It is problematic for the synchronous HARQ protocols if the associated feedback can not be transmitted from the eNB when the UEs expect to receive it.        In case the reference symbol transmission gap coincides with the transmission of the physical broadcast channel (PBCH) then, since the UEs may not be able to estimate the channel due to the lack of reference symbols, the broadcasted information may not be detectable by all UEs.        
It is notable that the negative side-effects listed above will degrade the performance of the system and ultimately also degrade the end user experience.